Method of maintaining water volume in an oil strata of an oil production reservoir

ABSTRACT

The volume of a water strata in an oil production reservoir is controlled by reinjecting separated water into that water strata. A production string is extended into a portion of a wellbore that extends laterally within the oil strata of the reservoir. A liquid flow including a hydrocarbon phase and a water phase is conducted from the oil strata into the laterally extending portion of the wellbore at a production zone. The water phase is separated from the liquid hydrocarbon phase within the production string, and then reinjected back into the water strata of the same reservoir from which it came, at a reinjection zone disposed below the production zone.

FIELD OF THE INVENTION

[0001] The invention relates to the separation of water from liquid hydrocarbons recovered from a production formation, and especially to a method of maintaining the volume of the water strata of the formation.

BACKGROUND OF THE INVENTION AND BRIEF DESCRIPTION OF THE RELATED ART

[0002] Hydrocarbon gases and liquids have been recovered from underground formations for over a hundred years. The recovery technology generally involves drilling a wellbore into the formation and withdrawing the hydrocarbons under reservoir pressure or by artificial lifting.

[0003] A typical production formation (see FIG. 1) consists of a trapped reservoir R which includes a gaseous hydrocarbon strata, a liquid hydrocarbon strata (oil), and a water strata. Gas, being lighter than oil, tends to accumulate above the oil. Water, being heavier than oil tends to accumulate below the oil, although the oil will contain certain quantities of water.

[0004] The reservoir R is pressurized from above by an overburden, i.e., the weight of rocks, soil, and fluids present above the reservoir. The gas is thus compressed and forces the oil strata downwardly against the water strata. The resulting pressure imposed on the oil strata drives the oil toward a wellbore W communicating with the oil strata. Thus, the presence of the water strata is important as it provides a support against which the oil strata is pressed by the gas strata to force the oil into the wellbore.

[0005] The oil leaving the oil strata through the wellbore is accompanied by water. The water, regarded as a contaminant, must be separated from the oil. The current technology involves separating the contaminants (e.g., water, carbon dioxide, nitrogen, hydrogen sulfide, helium, and other trace gases) from the hydrocarbon above ground, which is costly. For example, in order to be able to dispose of the separated water above the surface, the entire oil phase must be removed from the water. However, a more desirable solution is to reinject the separated water back into the ground, thereby eliminating the need to remove the entire oil phase from the water.

[0006] A particular benefit arises from returning the separated water/oil phase back to the same reservoir R from which it was withdrawn. In that regard, during oil production the water strata disposed within the reservoir R becomes gradually depleted. As a result, rather than being pushed by the gas out of the reservoir, the oil is pushed into voids created by the missing water. Thus, the pressure imposed on the oil strata is reduced, as is the recovery of oil. Since the oil recovery will terminate once the pressure becomes too weak to push out the oil, the risk of the hole being permanently unproductive is increased. Eventually, water may naturally migrate to the reservoir from remote areas, to repressurize the oil, but that could take a considerable period of time. Therefore, it would be desirable to maintain the water volume and thus oil pressure within a reservoir that is being depleted. That can be accomplished by pumping the separated water/oil phase back into the wellbore to a reinjection location within the water strata.

[0007] As pointed out earlier, the separation of water from hydrocarbons has traditionally been performed at the surface. However, the need to raise the water to the surface along with the hydrocarbons before separating the water increases the cost and complexity of the recovery operation. For that reason, below-surface separation techniques have been proposed wherein the water would be separated from the hydrocarbons at a location below the surface, and the separated water then reinjected (e.g., see U.S. Pat. No. 4,241,787). For example, as depicted in the accompanying FIG. 2, an oil/water mixture entering the wellbore W from a production zone 2 is conducted through a separator 3 located in the production string. The separated oil is conducted to the surface, and the separated water is reinjected into a disposal formation 4 at a reinjection zone 5 located directly beneath the production zone.

[0008] As noted earlier, it would be desirable to reinject the separated water into the water strata of the same reservoir from which the water came, in order to shorten, as much as possible, the distance that the water must be conducted, and to maintain the water volume within the reservoir so that hydraulic pressure acting on the oil strata is not excessively weakened. However, there could occur a tendency for the reinjected water to be drawn upwardly directly from the reinjection zone into the production zone (along phantom lines 6 in FIG. 2), rather than being dissipated within the disposal formation. That is, a so-called “coning” action caused by the pressure drawdown at the production zone tends to draw the reinjected water directly upwardly. The intensity of the coning action on the reinjected water would vary, depending upon various factors such as, for example, the density of the produced fluids, the formation's relative permeability to oil, water, and gas, the height of the reservoir R, the wellbore radius, the drainage radius, the distance between the wellbore perforations and the gas/oil interface and the oil/water interface. If the reinjected water were drawn directly to the production zone, then the fluids entering the wellbore would be heavily diluted with the reinjected water, thereby reducing production.

[0009] On the other hand, by locating the reinjection zone so far below the production zone that coning would not present an appreciable problem, a deeper and more costly wellbore must be drilled, and could result in the reinjection zone being spaced below the water strata and thus unable to maintain the water volume therein.

[0010] Therefore, it would be desirable to be able to separate water from oil at a subsurface location, and to reinject the separated water back into the water strata of the same reservoir from which it came, without the reinjected water being drawn directly back to the production zone by a coning action.

SUMMARY OF THE INVENTION

[0011] The present invention solves the above-described problem and involves a method of controlling the volume of a water strata of an oil production reservoir having a gas strata, a water strata, and an oil strata disposed below the gas strata and above the water strata. The method comprises the steps of:

[0012] A. extending a production string into a portion of a wellbore that extends laterally within the oil strata;

[0013] B. conducting a liquid flow including a liquid hydrocarbon phase and a water phase from the oil strata into the laterally extending portion of the wellbore and the production string at a production zone;

[0014] C. separating the water phase from the liquid hydrocarbon phase within the production string; and

[0015] D. reinjecting the separated water phase back into the water strata of the same reservoir from which it came, at a reinjection zone disposed below the production zone.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The objects and advantages of the invention will become apparent from the following detailed description of a preferred embodiment thereof in connection with the accompanying drawings in which like numerals designate like elements and in which:

[0017]FIG. 1 is a schematic view of an oil production reservoir;

[0018]FIG. 2 is a view of a conventional wellbore adapted to reinject waste water below a production zone;

[0019]FIG. 3 is a schematic view of a production zone and a water reinjection zone of an oil production reservoir, according to the invention;

[0020]FIG. 4 is an enlarged schematic view of the depicted in FIG. 3;

[0021]FIG. 5 is an enlarged sectional view taken through an oil/water separation section of a production string disposed in the wellbore; and

[0022]FIG. 6 is an enlarged sectional view through one of the separating elements depicted in FIG. 5.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0023] Schematically depicted in FIG. 3 is a wellbore lined with a well casing 12. The well casing 12 is bifurcated, i.e., a so-called dual lateral completion, into a main casing 12 a and a secondary casing 12 b extending laterally into an oil strata and a water strata, respectively, of a reservoir or production formation R. A production string 14 is suspended within the well casing and extends through the main casing 12 a and laterally (i.e., non-vertically—preferably horizontally) into the oil strata.

[0024] Extending within the secondary casing 12 b is a secondary line which comprises a tubing 17 extending laterally into the water strata to a location below the production string 14.

[0025] A mixture of liquid hydrocarbon phase, water phase and solids phase enters the production string 14 at a production zone 24 defined by suitable perforations formed in the main casing 12 a. The mixture travels uphole to a separation section 40 which separates the water phase from both the liquid hydrocarbon phase and the solid phase (see FIG. 5). The separated hydrocarbon phase, together with the solid phase, travels to the surface within the production string, to be recovered. The separated water phase emerges from the production string through a permeable segment 26 thereof and enters a collection zone 28 formed in the casing 12 between packers 22 and 30 (see FIG. 4). That water phase is transported at higher pressure downwardly through tubing 17 by a pump 32 and is injected back into the water strata of the reservoir R at a reinjection zone 27 defined by perforations formed in the secondary casing 12 b.

[0026] The present invention stems from a realization that a lateral wellbore requires a substantially smaller pressure drawdown to produce at the same rate as a vertical wellbore. The smaller pressure drawdown enables the reinjection zone to be located closer to the production zone without the reinjected water being drawn directly thereto by the weaker pressure drawdown. Thus, it is less likely that the reinjection zone will have to be located so far below the production zone that it may lie below the water strata of the reservoir in question. In other words, the reinjection zone can be located closer to the production zone of a lateral wellbore than to the production zone of a vertical wellbore. Consequently, the required depth of the reinjection wellbore is shorter, making it more likely that the separated water can be reinjected into the water strata of the same reservoir R from which it was removed, thereby helping to maintain the water volume and the oil pressure.

[0027] The reinjection zone is located below the production zone, but it need not be disposed directly beneath, i.e., in vertical alignment with, the production zone as shown. Instead, the reinjection zone but could be spaced horizontally from a vertical plane passing through the production zone.

[0028] The separation section 40 comprises separators in the form of identical hollow tubular membrane units 42, 44, 46, etc. arranged longitudinally in series within an outer separator casing 39 (see FIGS. 5, 6). As shown in FIG. 6, the membrane unit 42, for example, includes a perforated tube 42B and front and rear end caps 42C, 42D, each end cap having screw threads at both ends. Thus, the front end cap 42C has an external front screw thread 42E and an internal rear screw thread 42F. The rear end cap 42D has front and rear internal screw threads 42G, 42H. the perforated tube 42B has external screw threads at its opposite ends which are secured to the female screw threads 42F, 42G of the end caps 42C, 42D.

[0029] At least one cylindrical membrane 42J is disposed within the perforated tube 42B, and an inner perforated tube 42K is positioned inside of the membrane 42J. Thus, the membrane 42J is protected and reinforced on its inner and outer sides.

[0030] The membrane may comprise polymeric membranes or inorganic membranes, preferably combinations of both. Polymeric membranes are more applicable for finer separation, whereas inorganic membranes are more commonly used for microfiltration applications (i.e., separating particles that are larger than 0.1 micron). Examples of suitable polymeric membrane materials include materials sold by Amicon under the designations YM30 (regenerated cellulose) and XM50 (acrylic copolymer), and those sold by Hoescht/Celanese under the designations PES20H (PES blend), and PA20 (polyamide), and PS100HM (PS blend). Examples of suitable inorganic membranes include those sold by Alco (U.S. Filters) under the designation Membralox (alpha Al₂O₃), and those sold by Pall Filter under the designation Rigimesh (sintered metal composite).

[0031] In practicing the present invention, the production string 14 is positioned in the well casing 12, the production string including a plurality of the membrane units 42, 44, 46, etc. arranged longitudinally in series within the outer casing 39. The membrane units are shown as attached directly to one another, but it would be possible to connect other elements between successive membrane units, if desired, as long as those elements were able to transmit fluid from one membrane unit to the next.

[0032] A flowing mixture of liquid hydrocarbon phase, solid phase, and water phase is forced from the production formation into the production string at the production zone 24. The mixture flows uphole through a hollow center of an initially-encountered membrane unit 42, whereby the liquid hydrocarbon phase and the solid phase remain in the hollow center, and the water phase passes radially outwardly through the membrane unit and into the collection zone 28. Any water phase remaining in the mixture as the mixture exits the initially-encountered membrane 42, passes into the next membrane 44 where the above-described operation is repeated. Eventually, virtually all of the water phase is caused to separate from the hydrocarbon and solid phases, which remain in the hollow centers of the membranes. The remaining liquid hydrocarbon phase and solid phase are conducted uphole within the production string to be recovered at the surface.

[0033] The water phase is pumped down-hole from the collection zone 28 through the tubing 17 and is reinjected back into the water strata of the same reservoir R from which it came. As a result, the water volume is not materially reduced, so the pressure on the oil strata is generally maintained. Furthermore, since the water reinjected back into the water strata is the same water originally present therein, there is no risk of changing the chemistry (e.g., salinity) of the water strata.

[0034] Importantly, the production zone is defined by a lateral wellbore, which generates a weaker pressure drawdown than a vertical wellbore, so the reinjection zone can be located closer to the production zone, i.e., within the water strata of the same reservoir from which it was removed.

[0035] Although the present invention has been described in connection with a preferred embodiment thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. A method of controlling the volume of a water strata of an oil production reservoir having a gas strata, a water strata, and an oil strata disposed below the gas strata and above the water strata, the method comprising the steps of: A. extending a production string into a portion of a wellbore that extends laterally within the oil strata; B. conducting a liquid flow including a liquid hydrocarbon phase and a water phase from the oil strata into the laterally extending portion of the wellbore and the production string at a production zone; C. separating the water phase from the liquid hydrocarbon phase within the production string; and D. reinjecting the separated water phase back into the water strata of the same reservoir from which it came, at a reinjection zone disposed below the production zone.
 2. The method according to claim 1 wherein step D comprises conducting the separated water phase through a secondary casing emanating from the wellbore and extending laterally into the oil strata to the reinjection zone.
 3. The method according to claim 1 wherein step D comprises reinjecting the separated water phase into the reinjection zone disposal substantially beneath the production zone.
 4. The method according to claim 1 wherein step D comprises reinjecting the separated water into the reinjection zone which is spaced horizontally from a vertical plane extending through the production zone.
 5. The method according to claim 2 wherein step C includes conducting the liquid flow within the wellbore past a location where the secondary casing emanates from the wellbore and then through a separator wherein the water phase is separated from the liquid hydrocarbon phase. 